In wellbores, annular barriers are used for different purposes, such as for providing an isolation barrier. An annular barrier has a tubular part mounted as part of the well tubular structure, such as the production casing, which is surrounded by an annular expandable sleeve. The expandable sleeve is typically made of an elastomeric material, but may also be made of metal. The sleeve is fastened at its ends to the tubular part of the annular barrier.
In order to seal off a zone between a well tubular structure and the borehole or an inner and an outer tubular structure, a second annular barrier is used. The first annular barrier is expanded on one side of the zone to be sealed off, and the second annular barrier is expanded on the other side of that zone, and in this way, the zone is sealed off.
The pressure envelope of a well is governed by the burst rating of the tubular and the well hardware etc. used within the well construction. In some circumstances, the expandable sleeve of an annular barrier may be expanded by increasing the pressure within the well, which is the most cost-efficient way of expanding the sleeve. The burst rating of a well defines the maximum pressure that can be applied to the well for expanding the sleeve, and it is desirable to minimise the expansion pressure required for expanding the sleeve in order to minimise the exposure of the well to the expansion pressure.
When expanded, annular barriers may be subjected to a continuous pressure or a periodic high pressure from the outside, either in the form of hydraulic pressure within the well environment or in the form of formation pressure. In some circumstances, such pressure may cause the annular barrier to collapse, which may have severe consequences for the area which is to be sealed off by the barrier as the sealing properties are lost due to the collapse.
Current requirements for collapse ratings have lead to the use of increasingly higher expansion pressures. However, not only burst ratings are affected by increasing expansion pressures, also a variety of downhole tools may become ineffective or stop functioning under high pressures. Therefore, some wells have limited the allowed expansion pressure used in the well to protect tools and instruments present in the well. The problem may be circumvented by decreasing the thickness or strength of the expandable sleeve. However, this impairs the collapse rating.